<![CDATA[Water reservoir management in company facilities requires water level monitoring that can provide fast information and help prevent empty or overflow conditions. This study designs and simulates an Internet of Things (IoT)-based water reservoir monitoring and filling system using NodeMCU ESP32, HC-SR04 ultrasonic sensor, LCD, LED, buzzer, relay, and Blynk. Different from studies that report the implementation of physical devices, this study is limited to Wokwi simulations so that the results cannot be considered as field validation. The experimental design uses five reference distance points, namely 200, 150, 100, 50, and 2 cm, with 200 cm as the empty reference condition and 2 cm as the full reference condition. Evaluation is carried out using absolute difference, mean absolute error (MAE), root mean square error (RMSE), and mean absolute percentage error (MAPE), as well as Black Box functional testing of connections, sensor readings, indicators, pump control, and data transmission to Blynk. The simulation results show an MAE of 0.056 cm, an RMSE of 0.066 cm, and a MAPE of 0.337%. The system also demonstrates monitoring and control functions according to the simulation scenario. The contribution of this research is not in the novelty of the components, but rather in the integration of the monitoring-control flow tailored to the company's reservoir monitoring needs, the definition of measurement reference points, and quantitative evaluation during the simulation stage. Subsequent validation needs to be carried out using a physical prototype, repeated measurements, latency testing, packet loss, and operational resilience.]]>
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